1. Technical Field
One or more embodiments of the present invention relate to a signal processing device and a signal processing method for amplifying an inputted analog signal, converting the amplified signal to a digital signal for output, as well as a reception device, a transmission/reception device, a communication module and an electronic device including the signal processing device. Specifically, one or more embodiments of the present invention relate to a signal processing device for processing signals received from a light waveguide through a light receiving element.
2. Background Art
In recent years, an optical transmission technique using an optical transmission path (light waveguide) such as an optical fiber is used for a data communication technique that enables large capacity data communication at high speed. In an optical transmission system using such an optical transmission technique, when a digital electric signal (hereinafter simply referred to as “digital signal”) is inputted, a transmission side device drives a light emitting element such as semiconductor laser based on the inputted digital signal, generates an optical signal, and transmits the generated optical signal to an optical transmission path. The optical signal from the transmission side device is propagated through the optical transmission path and reaches the reception side device. The reception side device converts the optical signal from the optical transmission path to an analog electric signal (hereinafter simply referred to as “analog signal”) with a light receiving element such as a PD (Photo-Diode), appropriately amplifies the signal with an amplification unit, and then converts the amplified signal to a digital signal for output.
Therefore, an analog circuit for processing the analog signal and a digital circuit for processing the digital signal exist in the reception side device. Recently, forming the single processing circuits on one chip is demanded from the standpoint of miniaturization of the device, and the analog circuit and the digital circuit are demanded to be formed into one chip.
On the other hand, lowering of the drive voltage of the digital circuit is advancing with miniaturization of the digital circuit, and lower voltage of the digital signal, that is, lowering of the common mode voltage of the digital signal is advancing. For instance, in the standard of digital interface, the LVDS (Low Voltage Differential Signaling) (amplitude of about 400 mV, common mode voltage of about 0.9 to 1.2 V) is transitioning to SLVS (Scalable Low Voltage Signaling) (amplitude of about 200 V, common mode voltage of about 0.2 V) of smaller amplitude and lower voltage drive.
The usage band of the digital signal is rising and higher speed of the digital signal is advancing due to the lowering of the drive voltage of the digital circuit. However, the harmonic noise also increases with rise in the usage band. Furthermore, the switching noise in the digital circuit also increases with higher speed of the digital signal. Such a switching noise is a steep pulse-shaped noise and has a wide band spectrum.
As described above, if the analog circuit and the digital circuit are formed into one chip, the noise generated in the digital circuit may propagate and adversely affect the analog circuit. Thus, noise countermeasures such as avoiding the noise from going around (propagating) from the digital circuit to the analog circuit or reducing the noise component mixed to the analog signal need to be taken.    Patent Document 1: Japanese Unexamined Patent Publication No. 2004-135188 (date of publication Apr. 30, 2004)    Patent Document 2: Japanese Unexamined Patent Publication No. 2006-081141 (date of publication Mar. 23, 2006)